1. Field of the Invention
The present invention relates to a wavelength division multiplexing optical communication system and an optical amplifying device, for transmitting wavelength division multiplexed signal light including a plurality of optical signals of different wavelengths while amplifying the optical signals by the optical amplifying device, and more particularly to a wavelength division multiplexing optical communication system and an optical amplifying device in which signal light power per one wavelength to be output from the optical amplifying device is kept constant.
2. Related Art
It has been recently demanded to increase a capacity of optical communication system, with popularization such as of Internet and image transmission. To cope therewith, a wavelength division multiplexing (WDM) optical communication system has been put to practical use, and there has been promoted development such as increase of the number of wavelength division multiplexing.
FIG. 21 is a block diagram showing a general constitution of a WDM optical communication system for performing multi-repeating transmission, by collectively amplifying WDM signal light by an optical amplifying device.
The system in FIG. 21 is constituted of a transmitting side terminal station 1, a receiving side terminal station 2, an optical fiber transmission path 3 for connecting them with each other, and a plurality of (in the figure, two) optical amplifying devices 4 (optical repeating stations) provided on the way of the optical fiber transmission path 3.
The transmitting side terminal station 1 includes: a plurality of optical transmitters (E/O) 1A for outputting a plurality of optical signals of different wavelengths, respectively; a multiplexer 1B for wavelength division multiplexing the plurality of optical signals, and for outputting them as WDM signal light onto the optical fiber transmission path 3; and a post-amplifier 1C for amplifying the WDM signal light to a required level. The receiving side terminal station 2 includes: a preamplifier 2A for amplifying the WDM signal light transmitted through the optical fiber transmission path 3, to a required level; a demultiplexer 2B for dividing light output from the pre-amplifier 2A into a plurality of optical signals corresponding to respective wavelengths; and a plurality of optical receivers (O/E) 2C for receiving and processing the plurality of optical signals, respectively.
At each of the optical amplifying devices 4, the WDM signal lights transmitted through the optical fiber transmission path 3 are collectively amplified. Further, at each of the optical amplifying devices 4, total power of output light is monitored, and there is conducted an automatic level control (ALC) for controlling the operation of optical amplifying device 4 so as to keep a monitored value constant. By rendering an output controlling method of optical amplifying device 4 to be ALC in this way, repeating gain becomes independent for each span, thereby advantageously resulting in readiness of system design.
In constituting a WDM optical communication system making use of a plurality of optical amplifying devices 4 as described above, there is generally restricted a transmission distance of WDM signal light due to xe2x80x9cwavelength dependency of gainxe2x80x9d (gain deviation) of each of optical amplifying devices 4. As an optical amplifying device effective for suppressing such wavelength dependency of gain, the present applicant has proposed a constitution such as disclosed in OAA""98, WA2, pp.173-176, and OAA""98, MD1, pp.54-57.
Such an optical amplifying device effective for suppressing wavelength dependency of gain adopts a basic constitution wherein there is provided a two-stage constitution including an optical amplifying section of preceding stage and an optical amplifying section of succeeding stage, and a variable optical attenuator is inserted at a middle stage. In such an optical amplifying device, there is realized the ALC of output light, by operating the respective optical amplifying sections of preceding and succeeding stages under an automatic gain control (AGC) to thereby suppress gain deviation, and by simultaneously controlling an optical attenuation amount of the variable optical attenuator of middle stage corresponding to an output light level of the optical amplifying section of succeeding stage. Further, output setting level of ALC in the above optical amplifying device is controlled such that output light power per one wavelength is kept at a constant value even when the number of used wavelengths is changed. Concretely, the output setting level of ALC is set at mxc3x97Po, assuming that the number of used wavelengths is m and the output light power per one wavelength is Po. The number m of used wavelengths is obtained such as by a monitoring control signal to be sent from for example a monitoring system. By controlling the output setting level in this way, the WDM optical communication system becomes possible to operate as a system even when the number of used wavelengths is changed from one wavelength up to a maximum number of wavelengths.
Meanwhile, there exists an upper limit value for output light power per one wavelength which is allowed to be sent from an optical amplifying device onto an optical fiber transmission path. This upper limit value is determined due to nonlinear effect (such as self phase modulation (SPM) and cross phase modulation (XPM) of an optical fiber transmission path). In a conventional WDM optical communication system, output light power Po per one wavelength of an optical amplifying device has been designed to be a value close to such an upper limit value.
There will be now described light to be output from an optical amplifying device, in detail.
In a general optical amplifying device, there is caused spontaneous emission light (ASE light) as input signal light is amplified, and this ASE light is added to the signal light to be output. When the optical amplifying device is ALC operating such that output light power per one wavelength becomes a predetermined value Po [W], total output light power PTout [W] to be output from the optical amplifying device is given by the following equation (1):
PTout=Mxc2x7Po=PTinxc2x7G+2xc2x7nspxc2x7hxcexdxc2x7xcex94fxc2x7(Gxe2x88x921)xe2x80x83xe2x80x83(1)
wherein: PTin is total input light power [W] to the optical amplifying device; G is a gain of the optical amplifying device; nsp is a spontaneous emission coefficient of the optical amplifying device; hxcexd is a photon energy [J]; and xcex94f is a bandwidth [Hz] of the optical amplifying device.
In the equation (1), the first term of right side represents a signal light component (when input light includes ASE light such as at an optical amplifying device of a preceding stage, including an amplified component of the ASE light,), and the second term represents an ASE light component caused at the above-mentioned optical amplifying device. The total output light power PTout is controlled to be constant in an optical amplifying device which performs an ALC operation. Thus, when separately considering output light power Po per one wavelength by dividing it into a signal light component and an ASE light component, the ASE light component (the second term of the equation (1)) is an error component relative to a signal light component to be controlled to a constant level corresponding to the aforementioned upper limit value. Namely, the output light power Po per one wavelength can be represented by the following equation (2) derived from the equation (1):
xe2x80x83Po=PTinxc2x7G/m+2xc2x7nspxc2x7hxcexdxc2x7xcex94fxc2x7(Gxe2x88x921)/m=PTinxc2x7G/m+xcex94Poxe2x80x83xe2x80x83(2)
wherein xcex94Po=2xc2x7nspxc2x7hxcexdxc2x7xcex94fxc2x7(Gxe2x88x921)/m[W].
As described above, signal light power per one wavelength is decreased by xcex94Po than a predetermined level of output light power Po. Thus, in a WDM optical communication system as described above, there is such a problem that input signal light power to an optical amplifying device of next stage is decreased whenever WDM signal light is repeated and amplified at an optical amplifying device, resulting in degradation of optical S/N ratio at a receiving side terminal station. This optical S/N ratio represents a ratio of signal light and noise light (ASE light). Further, there is generally required an optical S/N ratio equal to or greater than a certain value, in order to achieve a predetermined code error rate at an optical receiver. The following equation (3) gives an optical S/N ratio when repeat and amplification is performed by optical amplifying devices of n stages:
1/OSNR=1/OSNR1+1/OSNR2+ . . . +1/OSNRnOSNRi=Pinsig(i)/(2xc2x7nsp(i)xc2x7hxcexdxc2x7xcex94f)xe2x80x83xe2x80x83(3)
wherein: OSNR is an optical S/N ratio after passing through an n-th optical amplifying device; OSNRi is an optical S/N ratio when an i-th optical amplifying device is used alone; Pinsig(i) is input signal light power into the i-th optical amplifying device; and nsp(i) is a spontaneous emission coefficient of the i-th optical amplifying device.
As apparent from the equation (2), as the number m of wavelengths is decreased, the reduction amount xcex94Po of signal light power per one wavelength is increased and has a larger effect on transmission characteristics. As an example, there has been obtained such an information that: in case of five-stage repeating, signal light power is only reduced by 0.5 dB upon usage of 32 wavelengths, whereas signal light power is reduced by as much as 12.3 dB upon usage of 1 wavelength . Generally, since an optical amplifying device used in a WDM optical communication system has broad-band characteristics, reduced amount of signal light power due to ASE light becomes significant upon usage of one wavelength.
To cope with such reduction of signal light power in case of a small number of used wavelengths, it is conceivable to perform a system design by for example previously estimating degradation of optical S/N ratio at the time of operation at a small number of wavelengths. However, in this case, there is caused such a problem that a system gain is reduced to thereby decrease a transmittable distance.
The present invention has been carried out in view of the conventional problems as described above, and it is therefore an object of the present invention to provide a WDM optical communication system and an optical amplifying device, adapted to prevent reduction of signal light power per one wavelength when repeating and transmitting WDM signal light making use of an ALC-operation optical amplifying device, to thereby achieve the improvement of transmission characteristic.
To achieve the above object, a wavelength division multiplexing optical communication system according to the present invention, including at least one optical amplifying device capable of collectively amplifying wavelength division multiplexed signal light; wherein output light of the optical amplifying device is controlled to be a predetermined output setting level; and wherein the wavelength division multiplexing optical communication system further comprises: a signal light output power controlling device for controlling an operation of the optical amplifying device so that signal light power per one wavelength included in the output light of the optical amplifying device is kept constant irrespectively of the number of wavelengths of signal light.
With the WDM optical communication system having such a constitution, the signal light power per one wavelength included in the output light of the ALC operating optical amplifying device, is kept constant by the signal light output power controlling device irrespectively of the number of wavelengths of the signal light, so that the signal light power of each wavelength is never reduced even when the WDM signal light is repeated and amplified via the optical amplifying devices, to thereby obtain excellent transmission characteristics.
As a first aspect of the WDM optical communication system, the signal light output power controlling device may comprise: an input light measuring section for measuring input light power of the optical amplifying device; a correction value calculating section for obtaining a noise light power caused at the optical amplifying device, based on: the input light power measured at the input light measuring section, a noise figure of the optical amplifying device corresponding to the input light power thereof, a bandwidth of the optical amplifying device, and the number of wavelengths of the signal light, so as to calculate an output correction value for increasing the output setting level of the optical amplifying device by the noise light power; and a correction executing section for executing a correction for the output setting level of the optical amplifying device, in accordance with the output correction value calculated by the correction value calculating section.
According to such a constitution, the correction value calculating section calculatingly obtains the noise light power caused at the optical amplifying device based on the input light power into the optical amplifying device obtained at the input light measuring section, the respective characteristics of the optical amplifying device and the number of wavelengths of the signal light; to thereby calculate the output correction value for increasing the output setting level of the optical amplifying device by the noise light power. Further, in accordance with the calculated output correction value, the correction executing section executes the correction of the output setting level so that the signal light power per one wavelength included in the output light of the optical amplifying device is kept constant irrespectively of the number of wavelengths.
As a concrete constitution of the first aspect, the input light measuring section and the correction executing section may be provided for each of a plurality of optical amplifying devices; and the correction value calculating section may be provided in the number of at least one for the plurality of optical amplifying devices; so as to collectively calculate the output correction values for the respective optical amplifying devices based on input light power, noise figure, bandwidth, and the number of wavelengths of the signal light, transmitted from each of the plurality of optical amplifying devices, to notify the thus calculated output correction values to the correction executing sections of the corresponding optical amplifying devices, respectively.
According to such a constitution, the output correction values for the plurality of optical amplifying devices can be collectively calculated by the correction value calculating section of such as a central station.
As another concrete constitution of the first aspect, the signal light output power controlling device may be provided for each of a plurality of optical amplifying devices; and each of correction value calculating sections of respective signal light output power controlling devices may be constituted to calculate an output correction value for the associated optical amplifying device, based on: an output correction value for an optical amplifying device at a preceding stage, and;
an input light power measured at the associated input light measuring section for the associated optical amplifying device, a noise figure and a bandwidth corresponding to the input light power, and the number of wavelengths of the signal light; so as to notify the thus calculated output correction value to the associated correction executing section, and to simultaneously transmit the thus calculated output correction value to a correction value calculating section of an optical amplifying device at a succeeding stage; so that output correction values are set sequentially from the optical amplifying device at an optical transmitting station side to the optical amplifying device at an optical receiving station side. According to such a constitution, the output correction values for the plurality of optical amplifying devices are calculated at the respective correction value calculating sections sequentially from the optical amplifying device at the optical transmitting station side.
Further, as a concrete constitution as modification of the first aspect, the signal light output power controlling device may comprise: a correction value storing section for storing output correction values which have been previously calculated corresponding to a combination of: the number of wavelengths of signal light which are predictable in the system, and the stage number of the applicable optical amplifying device from an optical transmitting station side; a setting notification section for notifying information concerning the number of wavelengths of the signal light at present and the stage number of the optical amplifying device from the optical transmitting station side, to the optical amplifying device; and a correction executing section for reading out the output correction value corresponding to the information from the setting notification section, from the correction value storing section, so as to execute a correction in accordance with the thus read out output correction value for the output setting level of the optical amplifying device.
According to such a constitution, the output correction values of the optical amplifying devices are previously calculated corresponding to a combination of: the number of wavelengths of signal light which are predictable in the system, and the stage number of the applicable optical amplifying device from an optical transmitting station side; and stored in the correction value storing section such as in a form of two-dimension correction value table. Further, the information concerning the number of wavelengths of the signal light at present and the stage number of the optical amplifying device from the optical transmitting station side, is notified from the setting notification section to the optical amplifying device, and the output correction value corresponding to the information is read out from the correction value storing section by the correction executing section, so as to execute the output correction. In this way, there is achieved simplification of the calculation processing for the output correction value.
As a second aspect of the aforementioned WDM optical communication system, the signal light output power controlling device may comprise: an optical measuring section capable of measuring at least one of: signal light power per one wavelength to be output from the optical amplifying device; and noise light power caused at the optical amplifying device; and a correction executing section for executing a correction of an output setting level of the optical amplifying device, based on the measuring result of the optical measuring section.
According to such a constitution, the signal light power per one wavelength to be output from the optical amplifying device or the noise light power caused at the optical amplifying device is actually measured by the optical measuring section, and the output setting level of the optical amplifying device is corrected based on an actual measuring result, so that the signal light power per one wavelength included in the output light of the optical amplifying device is kept constant irrespectively of the number of wavelengths.
As a concrete constitution of the second aspect, the optical measuring section may detect noise light power caused at the optical amplifying device, and the correction executing section may execute a correction for increasing the output setting level of the optical amplifying device by the detected noise light power. Alternatively, the optical measuring section may measure a spectrum of the output light of the optical amplifying device to thereby detect averaged signal light power per one wavelength included in the output light, and the correction executing section may correct the output setting level of the optical amplifying device so that the signal light power becomes a constant value.
Further, the optical amplifying device according to the present invention capable of collectively amplifying wavelength division multiplexed signal light of which output light is controlled to be a predetermined output setting level, comprises: a signal light output power controlling device for controlling the optical amplifying operation so that signal light power per one wavelength included in the output light is kept constant irrespectively of the number of wavelengths of signal light.
According to the optical amplifying device having such a constitution, the signal light power per one wavelength included in the output light under an ALC operation, is kept constant by the signal light output power controlling device, irrespectively of the number of wavelengths of the signal light. By applying such an optical amplifying device to a WDM optical communication system, the signal light power of each wavelength is never reduced even when the WDM signal light is repeated and amplified, to thereby obtain excellent transmission characteristics.